: Venezuelan equine encephalitis virus (VEE) is an important equine and human pathogen. It periodically emerges in epidemics caused by highly virulent epizootic strains. Virulence determinants of the natural VEE strains are not well understood at the molecular level. In vaccine strains and laboratory attenuated strains, virulence determinants for mice have been mapped to El and E2 glycoproteins and the 5' untranslated region (utr). A single change at nucleotide 3 (nt3) of the 5' utr of the VEE genome (G to A substitution) introduced into a molecular clone of wild-type virulent VEE results in a virus that is completely attenuated in mice. The long-term goal is to determine the mechanisms by which specific mutations in the 5' utr of the VEE genome attenuates the virus, and to use the VEE system to elucidate the role of lFNAB in alphaviruses pathogenesis. We hypothesize that the nt3A mutation results in a virus with increased sensitivity to the antiviral actions of lFNaB due to increased levels of viral dsRNA replication intermediates, consequently (a) increasing induction of IFN-stimulated antiviral genes, (b) enhancing activation of IFN-induced antiviral proteins that require dsRNA as cofactor, or alternatively, (C) the nt3A mutant virus may be slower in inhibiting host protein synthesis, allowing continued synthesis of key antiviral activities. We will test these hypotheses in the context of the following specific questions: 1) What is the relative contribution of INFaB-induced antiviral pathways PKR and 2'-5' OAS/RNase L in the inhibition of replication of wild type compared to nt3A mutant VEE in vitro and in vivo? 2) Does the nt3A mutation affect the induction and/or the activation of downstream IFNaB effector mechanisms such as PKR, 2'-5' OAS, RNase L, and ADAR? and 3) What is the effectof the nt3A mutation on virus-induced host protein synthesis inhibition?